A neighborhood near the school turned into a triage area for the wounded, and students who ran there spoke of their horrible experience. CNN affiliate reporter Hendrik Sybrandy, of KMGH, reports. (April 21)

LITTLETON, Colorado (CNN) -- Police said Wednesday that 15 people, including two teen-age suspects, were killed in America's worst instance of school violence. The bodies -- including those of the suspects -- were still inside Columbine High School in suburban Denver as police checked for possible bombs. At least one explosive device was found before dawn.

Tuesday's shooting rampage also left more than 20 people wounded, some of them critically.

After finding "about 10 to 11 explosive devices" in the school and its parking lot earlier in their investigation, police on Wednesday morning were methodically checking classrooms, discarded belongings and parked cars at the school.

"(The bomb squad doesn't) believe the building is secure," Jefferson County Administrator Ron Holliday told CNN. "In fact, they found another explosive device (about 5:30 a.m./7:30 a.m. EDT), so that is impeding the investigation considerably."

An earlier search for bombs ended at 2 a.m. One device detonated as the bomb squad was removing it late Tuesday night. No one was hurt.

Authorities identified Eric Harris, 18, and Dylan Klebold, 17, both juniors at the school, as the two gunmen wearing black trench coats who laughed and hooted on Tuesday as they opened fire on classmates and then killed themselves.

The two shooters were found dead in the library -- victims of what Jefferson County Sheriff John Stone called a "suicide mission" that involved booby-trapping the school with pipebombs.

Police said there is evidence of several explosions in the school's hallways and in other areas of the building.

After the siege ended, police said that as many as 25 people may have been killed. Early Wednesday, they revised the estimated death toll downward without elaborating.

Authorities have named Harris, left, and Klebold as the suspected gunmen

Officials say 12 bodies were found in the library -- nine males and three females.

Police say one adult body was found outside the library.

Three people were being detained in the case but they "have not been charged with anything at this point," Holliday said. "They are being questioned about what they may know about this incident."

Many stunned students, parents and residents of Littleton, an affluent Denver suburb, attended a memorial service Tuesday night, and school officials were arranging crisis counseling for teens struggling to cope with the massacre, the most recent of several school shootings nationwide.

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While police have not given a motive, several students said Harris and Klebold were members of a group calling itself the "Trenchcoat Mafia," outcasts who bragged about guns and bombs and hated blacks and Hispanics, as well as student athletes.

Police who searched Harris' home said they found bomb-making material. Students said the group was fascinated with World War II and the Nazis and noted that Tuesday was Adolf Hitler's birthday.

The attackers marched into the library of Columbine High School with guns and pipe bombs, demanding that "all jocks stand up. We're going to kill every one of you," said student Aaron Cohn.

A gunman looked under a desk in the library and said "Peek-a-boo," then fired, Cohn said. Anyone who cried or moaned was shot again. One girl begged for her life, but a gunshot ended her cries, the student said.

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Cohn said one killer put a pistol to his head but did not shoot him. Instead, he said, the shooter turned his attention to a black student, saying, "I hate niggers." Cohn heard three shots but couldn't see what happened.

"You could hear them laughing and running upstairs," said one student, who broke down in tears as she recounted the killing spree. "They didn't care who it was and it was all at close range."